1. Field of the Invention
The present invention relates generally to a drive unit for driving an electric motor by computer control based on information from various sensing devices, and more particularly to a motor drive unit for use with an electric power steering apparatus or a four-wheel steering system installed in a motor vehicle.
2. Description of the Related Art
An electric power steering apparatus installed in a motor vehicle includes a steering torque sensor for detecting a steering torque produced during steering operation by the driver. The steering torque sensor generates a torque signal corresponding to the detected steering torque, which is used in combination with a signal from another sensing device to generate a motor drive signal, such as PWM (pulse-width modulation) signal, for controllably driving an electric motor via a drive unit in such a way that an appropriate steering assist torque is supplied from the electric motor to the steering system, thereby lightening the manual steering effort of the driver.
One example of known motor drive unit includes a bridge circuit (H-type bridge circuit) composed of two pairs of switching elements or devices, such as junction field-effect transistors "JFETs" or metal-oxide-semiconductor field-effect transistors "MOSFET", each incorporating one diode connected in parallel with the associated switching device to block current flow in the opposite direction from that of the associated switching device (the diode being thereinafter referred to as "reverse blocking diode"). The switching devices in each pair are connected in series and two pairs of such series connected switching devices are connected in parallel. A power supply is connected to a junction between the two parallel connected switching device pairs, and the electric motor is connected between respective junctions between the series connected switching devices.
The motor drive unit of the electric power steering apparatus, as against a control unit for an automotive engine, does not employ a duplex system in which each of two distinct units becomes available to recover over a failure of the other unit. Accordingly, a failure occurring in the motor drive unit may result in malfunctioning of the entire system of the electric power steering apparatus. In the case where the failure is the so-called "Off fault" of any of the switching devices (at which the faulty switching device is fixed in the OFF state), no power is supplied from the power supply to the electric motor. In case of the failure being the so-called "ON fault" of any of the switching devices at which the faulty switching device is fixed in the ON state, power from the power supply is continuously supplied to the electric motor. In either case, accurate drive control of the electric motor is difficult to achieve.
The foregoing problems may be overcome by an improved motor drive unit in which a duplex system is incorporated. There still exist a problem however that when each of two distinct drive units is operating to controllably drive the electric motor while the other drive unit assumes a standby status, current may flow from the power supply to the other drive unit, deteriorating the accuracy of drive control of the electric motor. Additionally, when an ON fault occurs in one of the switching devices of the other drive unit, operation of one drive unit causes the other drive unit to form a closed circuit which allows the electric motor to act as a generator, making it difficult to achieve accurate drive control of the electric motor.